Study has been conducted on a sample analyzer which detects a detection target molecule by utilizing a magnetic particle. As the magnetic particle, use is made of a magnetic microparticle which contains a magnetic material such as magnetite and is formed with a grain size of several-ten nm to several μm. It is known that such a magnetic particle has superparamagnetism. If a magnetic field is applied, the magnetic particles have magnetism, and move by magnetic force acting along a gradient of the magnetic field. If the magnetic field is shut off, the magnetic particles lose magnetism and are dispersed. The magnetic material, which constitutes the magnetic particles, has a greater specific gravity than water. Hence, if the content rate of magnetic material is large, the magnetic particles sediment and precipitate in a sample. Thus, the magnetic particle is formed by combining a magnetic material with a high-molecular material, so as to have such a composition that the specific gravity is made close to that of water. Furthermore, in order that the magnetic particle can specifically bond directly or indirectly to a specific detection target molecule, the magnetic particle is formed by chemically modifying the surface of the particle with a material, such as an antibody, which specifically bonds to the detection target molecule.
As the sample analyzer utilizing the magnetic particles as described above, there is known, for instance, an apparatus which switches the direction of movement of the magnetic particles by mechanically moving at least one magnet relative to a sensor cartridge. Specifically, the sensor cartridge is disposed between a pair of magnets which are provided such that their magnetic poles are opposed, and the paired magnets are mechanically moved at the same time relative to the sensor cartridge. In addition, there is known an apparatus in which a permanent magnet that applies a magnetic field to a sample is inserted into and drawn out of a C-shaped magnet by a moving mechanism such as a rotary disc, in order to switch ON/OFF of the magnetic field.
In this apparatus, in the state in which the permanent magnet for applying a magnetic field is disposed between the magnetic poles of the C-shaped magnet, the magnetic flux is closed between these two magnets, and the magnetic field, which is applied to the sample, can be reduced to substantially zero. However, in this structure, it is necessary to dispose the C-shaped magnet at a position away from a reaction container, so as to prevent the magnetic field formed by the C-shaped magnet from affecting the magnetic field distribution in the sample. Thus, the magnetic field generator including the moving mechanism of the permanent magnet becomes larger. In addition, since the C-shaped magnet and the permanent magnet for applying a magnetic field attract each other by magnetic force, an operation mechanism, which can exert a stronger force than the attractive force, is needed when the magnetic field is switched from ON to OFF. Furthermore, in this structure, when the magnetic field is switched from ON to OFF, the permanent magnet for magnetic field application is moved in parallel to a sensing area. Thus, due to this switching, magnetic particles collected to the sensing area are also dragged and moved, and the distribution of magnetic particles in the sensing area is distorted. Consequently, the efficiency of bond-reactions of magnetic particles with the sensing area surface deteriorates.